[NETFILTER]: nf_nat_sip: fix NAT setup order

We need to set up the destination NAT mapping before the source NAT
mapping, so the NAT core gets to see the final tuple and can decide
whether the source port needs to be remapped.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>